This invention relates to nondestructive testing techniques and, more particularly, to techniques for ascertaining the surface properties of a material.
The surface properties of a material determine, to a great extent, the fatigue, wear, and corrosion characteristics of that material. Specific surface treatments, for example, may be applied to a material to inhibit deterioration and to extend the useful life of the material under adverse conditions. Because of this relationship between the surface properties and the structural characteristics of the material, the quantitative determination of surface properties has long been a goal of materials research.
Ultrasonic techniques provide one potential nondestructive approach for the measurement of surface properties. A surface acoustic wave is known to penetrate into a solid object to a depth which is roughly proportional to the wavelength of the wave. A multiple frequency wave will thus disperse (i.e., travel in the material with a frequency dependent velocity) in the presence of gradients of those physical properties which affect the velocity of the wave, such as the density and/or the elastic moduli of the object. This phenomenon of dispersive propagation makes it possible to sample the elastic properties of a material at various depths by measuring the velocity dispersion experienced by a surface acoustic wave in the material, that is, the variation of the propagation velocity as a function of the frequency of the wave. By performing an inversion of this dispersion data, the structure of the subsurface profile may then be deduced.
A known approach for obtaining such dispersion data is to inject acoustic tone bursts into the material at a limited number of discrete frequencies. This discrete frequency technique, however, yields relatively sparse dispersion data and, in addition, has been found to be too cumbersome and slow to be useful in practical applications. Consequently, a need has arisen for a new acoustic technique to quantitatively determine the surface properties of a material.